1. Field of the Invention
The present invention relates to a method and an apparatus for image formation, and more particularly to a method and an apparatus that can produce a high quality image by reducing an occurrence of an adhesion of a carrier to an image bearing member.
2. Discussion of the Background
Recently, a growing number of an image forming apparatuses adopt a reverse development system. In the image forming apparatus having the reverse development system, toner charged in the same polarity as that of an image bearing member, and a two-component developer including a carrier which is charged in a reverse polarity of the toner are used. An electrostatic latent image formed on a surface of the image bearing member is then developed into a toner image by applying a development bias having the same polarity as that of the toner to a developer bearing member which carries the two-component developer.
In the image forming apparatus, a pretreatment of an image forming operation (e.g., data transmission, and feeding of transfer sheet), an image forming operation (e.g., writing, development, and transfer of image), and an aftertreatment of the image forming operation (e.g., sheet discharging, and cleaning after image is transferred) are performed by driving the image bearing member.
In the above-described image forming apparatus using the two-component developer, when the image bearing member is started to drive, if the development bias is applied to the developer bearing member at the same time when the image bearing member is charged, an inconvenience, such as developing a solid black area on the image bearing member is caused because it happens that the image bearing member, passing through a developing section where the image bearing member and the developer bearing member face each other immediately after the development bias is applied, is not charged. Further, when the driving of the image bearing member is stopped, if the charging of the image bearing member is stopped at the same time when the application of the development bias is stopped, it happens that the image bearing member, which passes through the developing section immediately after the application of the development bias is stopped, is charged, resulting in an adhesion of the carrier to the image bearing member.
Thus, the time to charge the image bearing member and to apply a development bias to the developer bearing member is controlled so that the above-described inconvenience may not be caused at the developing section, when the image bearing member starts a driving or stops the driving. For example, as illustrated in FIG. 3, when the image bearing member is started to drive, an application of a development bias is controlled to be started xe2x80x9cTxe2x80x9d seconds after a charging of the image bearing member is started in which the image bearing member reaches the developing section from the charging section where the image bearing member opposes the charging device. Further, the application of the development bias is controlled to be stopped xe2x80x9cTxe2x80x9d seconds after the charging of the image bearing member is stopped, when the driving of the image bearing member is stopped. The time to charge the image bearing member and to apply the development bias is controlled as described above to obviate an occurrence of the above-described inconvenience.
Further, when an abnormal condition is encountered or when a transfer sheet is jammed while it is conveyed in the image forming apparatus, the charging and development bias application operations are not suddenly stopped. It is controlled such that the application of the development bias is stopped xe2x80x9cTxe2x80x9d seconds after the charging of the image bearing member is stopped as illustrated in FIG. 3 to prevent the occurrence of the above-described inconvenience.
It may happen that a user suddenly turns off a main switch of the apparatus or opens a door of the apparatus because of a production of an abnormal image, etc., while the image bearing member drives. In such a case, a supply of a voltage to a driving motor is stopped, and at the same time, the charging and development bias application operations are suddenly stopped, for safety. A rotation of the image bearing member stops after the image bearing member is rotated by a coasting of the driving motor, after the supply of the voltage to the driving motor is stopped. At this time, a carrier adheres to a portion of the image bearing member which passes the developing section by inertia because a development bias is not applied while the portion of the image bearing member is charged.
In addition, when a main switch of the apparatus is turned on or when the apparatus is activated by closing a door of the main body of the apparatus, it may happen that a portion of the image bearing member, which is kept being charged, passes the developing section before the development bias is applied. Then, the carrier may adhere to this portion of the image bearing member. The above-described actions (i.e., to turn off the main switch or to open the door of the main body suddenly while the image bearing member dives) may hardly be taken when it is obvious that a transfer sheet is inside the apparatus and an image forming operation is performed. However, these actions may often be taken when pretreatment or aftertreatment operations are performed in which a part or whole of a transfer sheet can visually be checked from the outside.
In recent years, it has become more common for an image forming apparatus is to be used as an output apparatus of a computing apparatus, such as a computer with a digitization of information. In the printer, it may happen that the image bearing member keeps on rotating due to an abnormal data input processing. When that happened, the printer is activated again after a power switch of the printer is turned off.
As is the case with the above-described image forming apparatus, a supply of a voltage to a driving motor is stopped; at the same time, the charging and development bias application operations are suddenly stopped. Thus, a carrier adheres to the image bearing member when a rotation of the image bearing member is stopped or when the printer is activated again.
When the carrier adheres to the image bearing member, an amount of the carrier decreases which may change a development characteristics. Further, the carrier adhered to the image bearing member may become caught in a gap between the image bearing member and a member that contacts a surface of the image bearing member (i.e. a cleaning blade of a cleaning device), thereby damaging the surface of the image bearing member and the cleaning blade. In addition, the carrier is conveyed to a fixing device via a transfer sheet where the carrier may damage a fixing roller, thereby seriously degrading a quality of an image.
The inventor proposed an image forming apparatus which prevents an adhesion of a carrier to an image bearing member in Japanese Patent Laid-Open Publication No. 5-165287. The adhesion of the carrier occurs when the image bearing member rotates by inertia after a supply of a voltage to a driving motor for the image bearing member is stopped while the image bearing member drives. The supply of the voltage to the driving motor is stopped when a main switch of the apparatus is turned off or a door of the apparatus is opened.
The image forming apparatus proposed in the above-described publication includes a delay control device which keeps on applying a development bias for a predetermined period of time, when the door of the a main body of the apparatus is suddenly opened. The apparatus further includes an internal power source and the delay control device. The internal power source supplies a power to apply the development bias, and the delay control device keeps on applying the development bias for the predetermined period of time, when the main switch is suddenly turned off.
An attempt has been made to accomplish a developing device using a two-component developer, in which a toner density of the developer is controlled to be within a constant range by taking toner into the developer with a movement of the developer without having a toner density detection device. As an example of the above attempt, the inventor proposed a developing device in Japanese Patent Laid-Open Publication Nos. 9-22178 and 9-197833. The developing device includes a regulating member to regulate an amount of a developer which is carried and conveyed by a developer bearing member, a developer container to contain a developer scraped by the regulating member, and a toner container, which is provided adjacent to the developer container, to supply the developer bearing member with the toner. In the developing device, a contacting state between the developer and the toner is changed by changing the toner density of the developer carried by the developer bearing member in order to change a state of the developer to take the toner.
In the developing device, when the toner density decreases as the toner is supplied to the developer carried by the developer carrying member from the toner container, a conveyance resistance of the developer in the developer container to the developer carried by the developer bearing member increases, thereby causing a pileup phenomenon in the conveyance of the developer. This pileup phenomenon forms a stagnation of the developer at a toner supply outlet through which the toner container is in communication with the developer container.
A supply of toner is regulated in a constant amount because the stagnation of the developer regulates an amount of the toner to be taken by the developer carried by the developer bearing member. The toner and carrier in the developer are stirred by a circulative movement of the developer in the developer container, resulting in an uniformed toner density. Thus, the toner density of the developer is self-controlled within a constant range by taking the toner into the developer with the movement of the developer without using the toner density detection device. Further, an agitating member, such as a paddle and a screw can be eliminated because the toner is taken into the developer with the movement of the developer, which is advantageous to downsize the developing device and to reduce costs of the developing device.
In the above-described developing device having a toner density self-control system, a target toner density can be controlled by changing a volume of a developer which is accomplished by changing an amount of a carrier contained in a developer container. More specifically, when the amount of the carrier decreases less than a certain level, the volume of the developer decreases. Thus, the density of the toner increases because an amount of the toner taken into the developer increases. To the contrary, when the amount of the carrier increases more than the certain level, the volume of the developer increases. Thus, the density of the toner decreases because the amount of the toner taken into the developer decreases.
Therefore, it is very much important to control an amount of the carrier such that the amount of the carrier contained in the developer container in the initial stage does not change in order to maintain the toner density within a proper range with respect to time. Further, the developing device having the toner density self-control system can be downsized compared to a developing device using a two-component developer in which toner is replenished by detecting a toner density using a conventional toner density detection device, and the developer is stirred in a developer container using a paddle or a screw. Therefore, an amount of a carrier to be contained in the developing device tends to be less.
In the developing device having the toner density self-control system, when the above-described carrier adhesion is repeated, the amount of the carrier in the developer container decreases. The toner density then increases which may produce an abnormal image, such as an image having a soiled background. In addition, an effect of a decrease of an amount of a carrier becomes more pronounced, since the amount of the carrier in the developer container is small.
The present invention has been made in view of the above-mentioned and other problems and addresses the above-discussed and other problems.
The present invention advantageously provides a novel image forming apparatus having a reverse development system and using a two-component developer, and method, wherein an occurrence of an adhesion of a carrier to an image bearing member is reduced to produce a high quality image, when a power switch of the apparatus is turned off or when a door of the apparatus is opened while the image bearing member is driven for an operation other than an image forming operation, or when the apparatus is activated again after the power switch is turned off.
According to an example of the present invention, an image forming apparatus includes an image bearing member, a charging device to charge an image bearing member, a latent image forming device to form an electrostatic latent image on a surface of the image bearing member, and a developing device having a reverse development system to develop the electrostatic latent image with a two-component developer including toner charged in the same polarity of the image bearing member and a carrier charged in a reverse polarity of the toner by applying a development bias having the same polarity of the toner to a developer bearing member which carries the two-component developer.